Communication systems operating at high radio frequencies, RF, e.g., above 5-7 GHz, usually convert information signals from baseband frequencies to high RF by using high frequency voltage controlled oscillators, VCO, which add phase distortion to the information signal.
Phase distortion is added both at the transmit side of the communication system, as the baseband information signal is up-converted to high RF prior to transmission, and also at the receive side of the communication system, as the high RF receive signal is down-converted back to baseband in order to recover the transmitted information signal.
Communication systems operating at lower RF, e.g., below 3 GHz, on the other hand tend to introduce less phase distortion than high RF communication systems, because of the lower frequency VCOs involved. Contrary to the communication systems operating at high RF, the signal processing architectures of these types of lower RF communication systems are rarely designed for handling large amounts of phase distortion, as this would add unnecessary complexity to the signal processing architecture.
High RF communication systems have been proposed for use in transporting the information signals of other communication systems, such as lower RF communication systems including mobile phone systems, mobile broadband systems, long term evolution, LTE, systems, and wireless local area networks, LAN, WiFi, WIMAX. Transporting here means that the information signal of the lower RF communication system is transmitted by the high RF system without regeneration of the modulated data in the information signal. This type of transport is at times referred to as “front-hauling”.
One way of transporting lower RF information signals over a high RF communication system is to up-convert the lower RF information signals to be transported either from baseband quadrature components or from a low carrier frequency (e.g. 2 GHz) up to the high RF. However, the VCOs used for up- and down-conversion will, as explained above, add distortion that in many cases will make the transmitted data impossible to recover by the lower frequency communication system signal processing architecture, since this architecture was not designed to cope with, e.g., high levels of phase noise.
Consequently, there is a need for distortion suppression in, e.g., high RF systems which does not require regeneration of data in the transported information signal.